The O.C. – Lindsey and Johnny because during their respective storylines, the show was way less enjoyable due to their scenes being so important. I can even give Oliver a pass even though I don’t really like the character/storyline but the first two characters really did suck the live out of the show.
Felicity Smoak on Arrow.
It took a little bit for her character to take over but the show suffered for it in the long run
Californication, his long lost son?? Ruined everything he and the show stood for.
Roy in The Simpsons. I was so glad when he moved out into his own apartment with those two sexy ladies.
Masuka’s daughter in dexter, she was so fucking useless
Edit: just to clarify, there were many so called “worse” characters. The point is that the daughter had absolutely no impact on the show. Ex. The other police officer going for sergeant created tension in the department, etc.